1. Field of the Invention
This invention relates to pipeline inspection tools and systems, and more particularly to apparatus and methods for above ground markers for use in connection with in-line tools.
2. Background of the Invention
Oil, petroleum products, natural gas, hazardous liquids, and the like are often transported using pipelines. The majority of these pipelines are constructed from steel pipe. Once installed, a pipeline will inevitably corrode or otherwise degrade. Proper pipeline management requires identification, monitoring, and repair of defects and vulnerabilities of the pipeline. For example, information collected about the condition of a pipeline may be used to determine safe operating pressures, facilitate repair, schedule replacement, and the like.
Typical defects of a pipeline may include corrosion, gouges, dents, and the like. Corrosion may cause pitting or general wall loss, thereby lowering the maximum operating pressure of the pipeline. Vulnerabilities may also include curvature and bending anomalies, which may lead to buckling, and combined stress and chemical or biological action such as stress corrosion cracking. Without detection and preemptive action, all such defects and vulnerabilities may lead to pipeline failure.
Information on the condition of a pipeline is often collected using an in-line tool. For example, an in-line inspection tool typically uses sensors to collect information about a pipeline as it travels therethrough. In the past, in-line inspection tools have used technologies such as magnetic flux leakage or ultrasonic testing to determine the condition of a pipeline wall. Flaws in ferromagnetic pipe can be detected by the perturbations they cause in a magnetic field applied to the wall of a pipeline. Flaws can also be detected by ultrasonic wall thickness measurement.
To collect useful data, the location and orientation of an in-line tool within a pipeline must be accurately known. When the location and orientation of an in-line tool are accurately known, then the locations of defects detected by the in-line tool can be accurately known. Accordingly, in-line tools often include components dedicated to determining location and orientation.
As an in-line tool travels through a pipeline, errors associated with the components measuring location may accumulate. These errors may reduce the accuracy with which an operator of the in-line tool can determine the locations of defects detected by the tool. Accordingly, markers are commonly placed outside the pipe at points with known locations. These markers communicate with the inspection tool to provide additional reference points that can be used to correct any errors associated with the onboard measuring components of an in-line tool.
Markers commonly locate in-line tools by recognizing the presence of the magnetic field emanating from the inspection system onboard the tool. However, this field is often weak and may be missed or misread by existing markers. Moreover, stray magnetic fields from extraneous sources such as electric power lines often appear the same as in-line tools to existing markers. Some markers include components intended to minimize the effects of stray magnetic fields. Such components make the markers large and typically do not work well.
Markers also commonly recognize the signal from transmitters onboard the in-line tool. Receivers are large and bulky and consist of a coil that is long and that makes the entire marker long. Attempts to make the receiving coil smaller have resulted in decreased sensitivity of the receiver and in markers that perform poorly.
Existing markers are not reliable because they often miss magnetic signals from in-line tools, they do not adequately discriminate between magnetic fields emanating from an in-line tool and stray magnetic fields, and they do not clearly recognize transmitted signals from in-line tools. Moreover, markers must be transported to reference locations but existing markers are large, bulky and difficult to transport. Accordingly, what is needed is a system and method that will provide required reference points more reliably and in a more compact package.